1. Field of the Invention
The present invention relates to a smoke detector for optically detecting contaminants such as smoke floating in the air.
2. Description of the Related Art
A smoke detector is used for preventing fire or as a detecting system at a time of occurrence of fire or in a semiconductor manufacturing plant or a food industry requiring a certain level of environmental conservation.
Conventionally, there are suggested various smoke detectors. In the following, as an example thereof, an outline of a “smoke detecting apparatus” disclosed in JP 11-23460 A will be described.
That is, the smoke detecting apparatus for determining fire by optically detecting smoke particles floating in the air sucked from a monitor space includes a laser diode which emits a laser beam having an electric field component in a predetermined direction, and an image formation lens for forming a light source image of an emission surface of the laser diode in a smoke detection region through which the sucked air passes. The smoke detecting apparatus also includes a light receiving element disposed on an optical axis that passes through an image forming position of the light source image in the smoke detection region, and is set on a surface substantially parallel to a direction of the electric field component of the laser beam, for receiving scattered light of the smoke particles, which has passed through the image forming position of the light source image and the vicinity thereof.
With this structure, in a case where there are suspended matters such as smoke particles in the air, the laser beam impinges upon the smoke particles to be scattered and the scattered light is received by the light receiving element. As a result, it is possible to sense generation of the smoke.
However, according to study of the inventor of the present invention, new problem is revealed regarding the detection of the smoke.
That is, generation of the smoke is detected by receiving the scattered light in a smoke detection portion by the light receiving element. However, when light other than the scattered light diffuses in an optical case, the diffused light is also received by the light receiving element. When an output signal of the light receiving element corresponding to the scattered light is represented by reference symbol S and an output signal thereof corresponding to the diffused light is represented by reference symbol N, an output in which the output signal S which is an original sensed signal is added with the output signal N which is a noise component is obtained.
In order to perform the smoke detection with high sensitivity, it is necessary that a level difference between the output signals S and N be made larger to increase an S/N ratio. In order to achieve this, intensity of light passing through the smoke detection portion has to be increased and diffusion of stray light has to be reduced.
On the other hand, the smoke detector is installed as a disaster prevention system in a building, a plant, or the like, so it is preferable that the smoke detector be small and light.